1. Field of the Invention
This invention relates to a magnetic bearing apparatus, and particularly to a magnetic bearing apparatus with a radial position correcting electromagnet for preventing a phenomenon of dragging on an auxiliary bearing that tends to occur in the vicinity of the resonance point of rigid mode (hereinafter called the first resonance point) on starting of a three axes control magnetic bearing apparatus.
2. Description of the Prior Art
On the basis of FIG. 8, a turbo-molecular pump in which a conventional three axis control magnetic bearing apparatus has been applied will be described. FIG. 8 shows a vertical sectional view of a magnetic bearing part of a three axis control turbo-molecular pump. Lower side permanent magnets 1A are disposed in the lower end of a frame 11 of the turbo-molecular pump. Upper side permanent magnets 1B face the lower side permanent magnets 1A across a small gap. The lower side permanent magnets 1A and the upper side permanent magnets 1B have different poles so that they attract each other. The upper side permanent magnets 1B are fixed to the lower end face of a metal disc 2. A collar 6 is disposed on the upper end face of the metal disc 2, and a shaft 5 passes through the central part of the metal disc 2, the upper side permanent magnets 1B and the collar 6. Axial electromagnets 3 are constituted so that they support the metal disc 2 and the shaft 5 in midair. An auxiliary bearing 4 is provided to prevent damage caused by mechanical contact between the shaft 5 and the axial electromagnets 3 and other elements occurring in the case of breakdown of the electromagnets.
While the pump is running, the magnetic bearing part supports the shaft 5 in midair by means of a balance of the force of the axial electromagnets 3 attracting the metal disc 2 and the attracting force between the lower side permanent magnets 1A and the upper side permanent magnets 1B.
However, in a conventional three axis control turbo-molecular pump, a phenomenon of vibration of the shaft 5 becoming large when the speed reaches the vicinity of the first resonance point upon start-up has sometimes been produced.
In a three axis control turbo-molecular pump, vibration of the shaft 5 is usually controlled by the attractive force between the lower side permanent magnets 1A and the upper side permanent magnets 1B. However, because with the attractive force of permanent magnets the rigidity is weak, there has been a danger of it not being possible to stably absorb vibration occurring at the first resonance point. In this case, it sometimes happens that the shaft 5 and the auxiliary bearing 4 accelerate while still in contact with each other (hereinafter called the drag phenomenon). This phenomenon most readily occurs due to a cause such as impurities entering inside the turbo-molecular pump thereby causing the amount of unbalance of the shaft 5 to have increased. When the shaft 5 and the auxiliary bearing 4 are continuously in contact with each other, the durability of the auxiliary bearing 4 falls. Also, the contact noise at this time is considerably large and the speed sometimes does not rise as far as the rated speed.